1. Field of the Invention
This invention relates to reticles, and particularly, reticles incorporated in instruments utilized for stereoscopic viewing of, for instance, aerial photographs.
2. Description of the Prior Art
The sensation of depth results from the fact that a human has binocular vision and does not see exactly the same view of an object out of each eye. A photograph is essentially a monocular view of the object being examined and, accordingly, has no depth. However, by taking two pictures of the object with a camera which is moved slightly from a first position to a second position, two slightly different pictures of the object are obtained. By examining these two pictures with a device known as a stereoscope, each eye sees only one of the pictures and, therefore, the same sensation of depth is obtained as if the original object was viewed directly.
In a typical application, two photos of the same object, in either transparency or print form, are viewed with a stereoscope and certain characteristics of the photos are analyzed and interpreted, as for example to identify the size or heights of objects. The well known principle of parallax allows very accurate object height information to be extracted from the stereo photos, provided certain photo parameters are known, e.g. flying height, camera focal length, etc. Implementation of this principle requires that a reticle be positioned on conjugate image points on each photo of the stereo pair, i.e. identical image points on each photo. By moving one reticle along the flight line or direction of flight, the reticle mark (usually in the form of a circular dot) will appear to rise or lower above or below the three dimensional view provided by the stereo photographs. When the dot appears to be placed in contact with the object, it can be said the operator has cleared parallax about the object of interest. By measuring difference in parallax between the object in question and an adjacent point, the relative height of the object can be readily calculated. This operation is known in photogrammetry as clearing parallax by the floating mark technique.
A problem exists, however, when the usual and well known reticles are used in the parallax measuring process. As the reticle pattern is generally dark and/or opaque, it becomes quite difficult for the operator to quickly and easily find it in the field of view should the area of the photo under study be mostly dark, or populated with a plurality of dark objects. The reticle pattern blends in with the background. This is troublesome in instruments having a low power of magnification, such as 1.times. to 3.times., where the reticle utilized generally comprises a dot of, perhaps, 0.5 mm in diameter. The problem is more severe in higher viewing magnifications since the actual reticle size is even smaller, and may be on the order of 0.01 mm to 0.02 mm in diameter. In these instances, dust which may have gathered on the photo will act to confuse the operator who must determine which among the field of dark and/or opaque dots on the photo is the actual reticle pattern.
The problem is further complicated, for instance, in instruments where the reticles are moved relative to the photograph. Accordingly, the reticle patterns are not generally centered in the field of view at the optical axis of the instrument. In fact, because the reticle is movable with respect to the photo, it would be most unlikely to be positioned at the center. Therefore, the operator must search the entire field of view to try and determine where the reticle pattern is actually located. This can be a most time consuming procedure resulting in considerable eye fatigue for the operator.
In an attempt to deal with the foregoing problems, some photogrammetric equipment manufacturers added arrows or other identifying marks to their reticles. The purpose of the arrow is to point to the location of the reticle pattern itself. However, this only aids the operator in determining the approximate position of the pattern. Also, the addition of the arrow introduces a new problem in that the arrow blocks adjacent portions of the photo.
Another attempt at solving the above described problems included painting the reticle pattern with a brightly colored non-fluorescing paint. However, this has not proven satisfactory either, as in order to make the color visible to the operator it is necessary to subject the reticle to illumination from above, so that the color of the reticle is reflected into the optics of the viewing instrument. Unfortunately, this illumination is also reflected into the optics of the viewer, thereby reducing image contrast and overall image quality. Further, the amount of light reflected from the reticle itself is small and the result is generally unsatisfactory because the reticle appears dark or, at least, dimly illuminated.
It will be appreciated from the foregoing description that there has been a long felt, but unsatisfied need in photogrammetry to provide a low cost, easily manufactured reticle pattern which is highly visible and easily recognizable by the operator in the field of view.